The invention relates to digital television receivers for vestigial-sideband (VSB) digital television (DTV) signals and, more particularly, to the portions of such receivers used for recovering baseband symbol coding proceeding from intermediate-frequency signals.
Bandpass trackers for VSB DTV signal receivers are described by C. B. Patel and the inventor in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d. An in-phase synchronous detector synchrodynes digitized VSB DTV signal to baseband for recovering symbol coding. U.S. Pat. No. 5,479,449 describes a narrow bandpass filter being used to convert the digitized VSB DTV signal to a digital narrow-band double-sideband amplitude-modulation (DSB AM) signal. A quadrature-phase synchronous detector synchrodynes the narrow-band DSB AM signal to baseband for recovering an automatic-frequency-and-phase-control (AFPC) signal for a local oscillator that generates local oscillations used in detecting the VSB DTV signal for digitization. U.S. Pat. No. 5,479,449 specifies that the response of the bandpass filter is centered on the carrier wave frequency and is narrow enough in bandwidth that the modulation of the carrier wave is suppressed respective to the carrier wave itself in the response.
U.S. Pat. No. 5,479,449 also describes alternative bandpass trackers for VSB DTV signal receivers which do not use a narrow bandpass filter to convert the digitized VSB DTV signal to a digital narrow-band DSB AM signal, but instead extract AFPC signal for the local oscillator from the response of a narrowband lowpass filter to the quadrature-phase component of the result of complex synchrodyning of the VSB DTV signal to baseband. This alternative type of bandpass tracker has been favored because the narrowband filtering of AFPC signal can be carried out in the analog regime, without employing a digital filter of considerable complexity.
There is a problem with bandpass trackers for VSB DTV signals that arises because the transmitted VSB DTV channel response rolls off in a band of frequencies flanking the carrier, but the transmitted VSB DTV signal does not use a modulation form free of components in a narrow band of frequencies flanking the carrier. This results in asymmetry of the in-phase sidebands near the carrier, so quadrature-phase synchronous detection will not be completely non-reponsive to these sidebands. Consequently, very low frequency components of modulation will at times appear in the AFPC signal for the local oscillator that generates local oscillations used in detecting the VSB amplitude modulation to recover baseband symbol coding. The resulting modulation of the frequency and phase of the local oscillator will manifest itself as undesirable intersymbol interference in the recovered baseband symbol coding.
The invention is embodied in certain digital radio receivers for a baseband symbol code that is designed to have a spectrum reaching esentially to zero-frequency and that is transmitted by a digital radio signal with vestigial-sideband amplitude modulation of a suppressed carrier wave, so as to have an asymmetrical channel response even in a narrow frequency band including a pilot carrier signal at the frequency of the suppressed carrier wave. Each of these receivers includes a local oscillator for generating local oscillations used in detecting the vestigial-sideband amplitude modulation, the frequency and phase of which local oscillations are controlled in response to an automatic-frequency-and-phase-control (AFPC) signal. Each of these receivers further includes a frequency-and-phase detector for generating the AFPC signal responsive to the pilot carrier signal. Each of these receivers is characterized by an automatic frequency and phase control loop connection of its frequency-and-phase detector and local oscillator, which loop connection includes a filter preceding its frequency-and-phase detector for reducing asymmetry of channel response in the narrow-frequency band including the pilot carrier signal. This prevents the appearance of very low frequency components of modulation in the AFPC signal for the local oscillator that generates local oscillations used in detecting the VSB amplitude modulation to recover baseband symbol coding. Accordingly, a cause of undesirable intersymbol interference in the recovered baseband symbol coding is substantially removed.